The objective of the research plan is to understand an aspect of cellular response to radiation or chemical injury. Cloned DNA sequences will be introduced into normal and radiation-sensitive cells by DNA-mediated gene transfer after exposure to radiation or chemicals. The genetic survival of these transfected DNA sequences will be assessed. Cellular DNA repair mechanisms and pathways of genetic recombination will be studied inmammalian cells as they are involved in reactivation of transfected, cloned genes. We have shown differential cytotoxic responses to UV and chemicals in DNA repair-proficient and -deficient Chinese hamster ovary (CHO) cells. We have also observed that different radiation-sensitive, repair-defective CHO mutants respond differently to DNA damage in transfected genetic sequences. We wish to know the molecular basis of this difference in response. We plan to devise and use assays for homologous and non-homologous genetic recombination to explore the basis for these differences. We also plan to introduce specific chemical lesions into cloned genetic sequences to ascertain whether they are reactivated differentially by repair mutants in transfection assays. The knowledge we expect to gain is directly relevant to participationby radiation and chemically-induced alterations of cellular physiology in genetic mechanisms underlying carcinogenesis.